Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. Also shown is top contact 24.
Conventional contacts 11 and 24 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO.
The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic. Further, other versions of the conventional MTJ 10 might include an additional pinned layer (not shown) separated from the free layer 20 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. Although shown as in-plane, the magnetization 21 of the conventional free layer 20 may have a perpendicular anisotropy. Thus, the pinned layer 16 and free layer 20 may have their magnetizations 17 and 21, respectively oriented perpendicular to the plane of the layers.
To switch the magnetization 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the top contact 24 to the bottom contact 11, the magnetization 21 of the conventional free layer 20 may switch to be parallel to the magnetization 17 of the conventional pinned layer 16. When a sufficient current is driven from the bottom contact 11 to the top contact 24, the magnetization 21 of the free layer may switch to be antiparallel to that of the pinned layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
When used in STT-RAM applications, the free layer 21 of the conventional MTJ 10 is desired to be switched at a relatively low current. The critical switching current (Ic0) is the lowest current at which the infinitesimal precession of free layer magnetization 21 around the equilibrium orientation becomes unstable. For example, Ic0 may be desired to be on the order of a few mA or less. In addition, a short current pulse is desired to be used in programming the conventional magnetic element 10 at higher data rates. For example, current pulses on the order of 20-30 ns or less are desired.
Although not shown, the conventional MTJ 10 could be a dual MTJ. In such a case, the conventional MTJ 10 would include an additional conventional barrier layer and an additional conventional pinned layer. The conventional free layer 20 would reside between the barrier layers. The additional barrier layer would be between the additional conventional pinned layer and the conventional free layer 20. In such a conventional dual MTJ, the conventional barrier layer 18 would typically be the same thickness as or thicker than the additional conventional barrier layer (not shown). Conventional dual MTJs generally have an improved switching current and symmetry, a smaller process margin, lower tunneling magnetoresistance (TMR), high resistance area product (RA), and a magnetic tunneling junction switching asymmetry that does not match the transistor supply current. In addition, a read disturbance for a normal layout may also occur.
Although the conventional MTJ 10 and a conventional dual MTJ may be written using spin transfer and used in an STT-RAM, there are drawbacks. For example, for conventional MTJs 10 having the magnetizations 17 and 21 oriented perpendicular, the magnetoresistance may be lower than a conventional MTJ 10 having its magnetization in-plane. Further, as discussed above, conventional dual MTJs may have a lower magnetoresistance than a single MTJ. As a result, the signal from the conventional MTJ 10 may be lower than desired. Such perpendicular conventional MTJs 10 also exhibit high damping. As such, switching performance is adversely affected. Thus, performance of a memory using the conventional MTJ 10 is still desired to be improved.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.